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Borrowers are unaffected so long as the base rate remains above the collar level, which changes among lenders.

However, interest rates are widely predicted to fall to below 2 per cent next year, and even as far as 1 per cent, according to some economists.

Darlington Building Society offered a collar of 4.5 per cent on the last two tracker products it offered before they were withdrawn last month.

It means borrower on these deals will not benefit from the full 1.5 percentage point cut.

A collar is the exact opposite of the better known cap on a mortgage. While a cap protects the borrower because it means a mortgage rate will never goes above a certain rate, a collar protects the lender because the mortgage rate is guaranteed not to go below a certain level.

Andrew Montlake, of mortgage borrowers Cobalt Capital, said: "If a lender makes it clear what the collar is, then this is not unfair. What is unfair however is when a lender tries to sneak a collar into their deal or when they leave a woolly translation.

"Changing the conditions half way through the term of the product means it does not end up doing what it says on the tin and this is unfair. We are likely to see more lenders introduce them."

A spokesman for Halifax said: "The 3 per cent bank rate condition is simply a power, we do not have to use it, so it is misleading to call it a collar in the sense that a collar is usually a specific floor built into a product.

"The condition simply provides us with an option that we may, or may not, choose to exercise. We are only permitted to use the power if the base rate falls below 3 per cent."

A spokesman for Darlington Building Society, said: "Borrowers are advised of all terms and conditions before they take any product on. These products didn't form a major part of our portfolio and are no longer open to new business."